Recently, great attention has been directed toward the synthesis of ABA triblock copolymers that function as thermoplastic elastomers. These materials are composed of long blocks of different homopolymers possessing incompatible aggregation behavior. Generally, two blocks are structurally rigid and dispersed in the form of minute glassy domains in a third block, usually a flexible polymer possessing an amorphous, rubbery phase. For example, FIG. 1 shows a schematic representation of the microphase behavior of SBS triblock copolymers, showing areas of aggregation of the glassy poly(styrene) blocks, dispersed in the amorphous poly(butadiene) chains. The glassy regions serve as anchors that hold the soft, elastomeric domains together in a network structure and effectively behave as crosslink points, eliminating the need to vulcanize the material. Heating thermoplastic materials above the melting (xe2x80x9cTmxe2x80x9d) or glass transition temperature (xe2x80x9cTgxe2x80x9d) of the xe2x80x9chardxe2x80x9d blocks softens the glassy domains and allows the copolymer to flow; while cooling returns the phase separation and the material again behaves as a crosslinked elastomer.
Poly(styrene)-b-poly(butadiene)-b-poly(styrene) (xe2x80x9cSBSxe2x80x9d) triblock copolymers are well-known thermoplastic elastomers. While their tensile strength properties are similar to that of natural rubber, they are dependent on a high degree of 1,4- over 1,2-poly(butadiene) chain microstructure for optimal elastomeric behavior. 
The most common synthesis of SBS involves a sequential addition anionic polymerization method (Scheme 1), which inherently introduces varying degrees of 1,2-poly(butadiene) content into the polymer backbone. 
The useful service temperature range of SBS triblock copolymers is ultimately determined by the melting temperature of the poly(styrene) (xe2x80x9cPSxe2x80x9d) domains. It has been shown that the strength of these SBS polymers drops sharply above 60xc2x0 C. as the Tg of the PS domains is approached. The use of end-blocks with a higher thermal resistance might provide a valuable answer to this problem. A potential candidate is poly(methyl methacrylate) (xe2x80x9cPMMAxe2x80x9d) since it exhibits a Tg of ca. 130xc2x0 C. (when its syndiotactic content reaches 80%). 
Unfortunately, the synthesis of poly(methyl methacrylate)-b-poly(butadiene)-b-poly(methyl methacrylate) (xe2x80x9cMBMxe2x80x9d) triblock copolymers is not as straightforward as SBS. A key problem is the inability of poly(methyl methacrylate) anions to initiate the polymerization of butadiene. Since butadiene anions are sufficiently nucleophilic enough to react with methyl methacrylate, attention has been directed towards the synthesis and use of difunctional initiators. However, this has resulted in marginal success and only recently have well-defined MBM triblock copolymers been synthesized. Unfortunately, these copolymers display a high content 1,2-poly(butadiene) microstructure content ( greater than 45%) and thus exhibit poor elastomeric properties.
As a result, a need exists for a synthetic methods which would allow the synthesis of such ABA triblock copolymers such as poly(methyl methacrylate)-b-poly(butadiene)-b-poly(methyl methacrylate) which exhibit good elastomeric properties.
The present invention relates to novel ABA triblock co-polymers that function as thermoplastic elastomers and methods for preparing the same. In general, the inventive ABA triblock polymers are prepared using a ring opening metathesis polymerization (xe2x80x9cROMPxe2x80x9d) reaction followed by an atom transfer radical polymerization (xe2x80x9cATRPxe2x80x9d) reaction. As it will be further disclosed below, this tandem approach allows for the synthesis of novel ABA triblock polymers which were not previously possible using prior art techniques. Briefly, ROMP is used to synthesize a telechelic polymer with end groups which function as ATRP initiators in the following manner: 
wherein:
n is an integer; 
is a cycloalkene;
Zxe2x80x94Yxe2x95x90Yxe2x80x94Z is a chain transfer agent wherein Z is a end group which functions as a ATRP initiator and xe2x80x94Yxe2x95x90Yxe2x80x94 is an alkenyl group; and, 
is the resulting telechelic polymer.
The ROMP reaction is followed by a ATRP reaction wherein the ROMP telechelic polymer product is further polymerized in the following manner: 
wherein:
m is an integer; 
is an alkene; and 
is the resulting ABA copolymer.